As exploration in the oil and gas industry has expanded, increased safety and environmental concerns have caused oil and gas exploration and drilling contractors to require an intermediate protection casing to be used between the surface casing and the conventional intermediate casing. Casing hangers of this type must provide sufficient weight carrying capacity with their support mechanisms while maintaining a pressure capacity comparable to that of the casing being suspended. Typically, hangers have failed in one way or another to meet these criteria. Often their use requires that the bore of the previous casing hanger be unduly restricted if a mandrel shoulder type hanger is used while the use of an expanding type hanger often requires undue restrictions in the annulus between the protection and surface casings, causing problems during cementing and circulating operations.
This invention is for an improved wellhead casing hanger particularly suited for use in situations where the annular spacing between successive casing strings is inordinately small. The present invention provides a unique wellhead hanger system which provides improved weight capacity, increased flow return area and full bore access to the protection casing below the hanger.
Prior casing hangers used in situations where the annular spacing between successive casing strings is inordinately small include two types of devices. The first type of these is disclosed by U.S. Pat. No. 3,421,580 which shows an expanding type hanger to suspend the protection casing. The expanding type hanger lands in a specially profiled circumferential groove in the surface casing hanger with flow return passages formed in the wall of the surface casing hanger. A similar structure is found in U.S. Pat. No. 3,847,215 wherein the expanding type hanger is used to suspend multiple tubing strings with flow return passages formed in the surface casing hanger in which it is landed.
The second type of casing hanger is shown in U.S. Pat. No. 3,592,489 wherein a shouldered or mandrel type hanger lands on a circumferential seat or shoulder protruding from the surface casing hanger's bore. Flow return passages are formed in the surface hanger's wall with the protruding shoulder split into a plurality of arcuate segments which are radially movable by piston means. A similar type of hanger is manufactured by FMC Corporation and is shown in the Composite Catalogue published by World Oil Publishing, '88-'89 Edition, Volume 2, p. 1497. The FMC structure differs from the structure of U.S. Pat. No. 3,592,489 by having the protruding shoulder an integral part of the wellhead housing.